The invention relates to a method for controlling the ignition time in internal combustion engines.
Modern internal combustion engines 1 (FIG. 1) have electronic ignition systems which use a crankshaft angle sensor 2 to measure the angular position of a crankshaft 3 with regard to the upper dead center of the piston. These ignition systems determine the ignition time by means of a predetermined angular position of the crankshaft, i.e. ignition of a fuel/air mixture which has previously been fed into the combustion space 4 is triggered at the predetermined angular position. These ignition systems are as a rule a component of an electronic control device of the internal combustion engine which also controls the feeding in of the fuel/air mixture which is fed to the combustion space 4 at an xe2x80x9cearlierxe2x80x9d or preliminary angular position.
In intake engines and conventional low-pressure injection devices, the time period which is necessary for feeding in the fuel/air mixture corresponds to a considerable crank angle range. The time for the feeding in of the mixture is triggered, as is the ignition time, at a predetermined angular position of the crankshaft, but in simple control devices the time of the start of the feeding in of the mixture is not determined as precisely as the ignition time, since it is assumed that the combustion is influenced essentially by the ignition time and, owing to the long period of feeding in fuel, it is not necessary to determine precisely the start of the feeding in of fuel.
In internal combustion engines 1 having an injection device 10, it is customary to use a single control de vice 9 to control both the injection process and the ignition time. In high-pressure injection devices, the fuel, or the fuel/air mixture, is fed to the combustion space 4 in significantly shorter time ranges so that, when such high-pressure injection devices are used, in particular with a direct injection of the fuel into the combustion space 4, the time of the injection process, tog is determined precisely and is triggered at a predetermined angular position of the crankshaft. The precise measurement of the angular position of the crankshaft, both for the injection process and for the injection time, constitutes a considerable computational outlay for the control device 9, the measurement process having to be carried out twice within a very short time.
High-pressure injection devices which operate according to the energy storage principle are known, for example, from WO 92/14925 and WO 93/18296. These high-pressure injection devices are used to inject the fuel into the combustion space in very short pulses. In addition, there are high-pressure injection devices which operate according to the solid-state energy storage principle, so-called xe2x80x9cPNS (pump/nozzle systems) injection devicesxe2x80x9d which are described, for example, in the German Patent applications P 195 15 781 and P 195 15 782. These PNS injection devices are provided in particular for directly injecting the fuel into the combustion space, it being possible for the time period for an injection process during idling to be shorter than half a millisecond. The crankshaft angle sensor 2 which is used for measuring the angular position of the crankshaft is composed of a toothed disk 5 with teeth 6 arranged on the circumference, and a sensor element 7 which is arranged at the circumferential region of the toothed disk 5 and senses the passing through of the teeth 6 and converts it into an appropriate electrical pulse signal. The electrical signal is fed to the control device 9 with a line 8. At high rotational speeds, the signal of the crankshaft angle sensor 2 has a high time resolution since the time intervals between the passing through of two successive teeth 6 at the sensor element 7 are very short. On the other hand, these time intervals are long at low rotational speeds (for example  less than 2000 rpm), such as occur during idling, so that the measurement of the rotational speed becomes imprecise, in particular if the rotational speed changes during the measurement, since a change in the rotational speed between the passing through of two successive teeth 6 cannot be sensed precisely using the crankshaft angle sensor 2. Such changes in rotational speed occur, for example, during unsmooth idling, so that the measuring errors in the measured rotational speed do not yet solve in an optimum way the problem of the correction to an idling desired-rotational-speed. The measuring errors cause high fuel consumption resulting in relatively high emission of pollutants.
FIG. 2 shows the rotational speed of an internal combustion engine [rpm] over the time period of one crankshaft revolution [s]. It emerges from this that at low speeds changes in the rotational speed bring about a large change in the revolution time and thus great inaccuracies in the measurement of the rotational speed and large measuring time errors.
U.S. Pat. No. 3,892,207 discloses an internal combustion engine which has a control device for driving the injection process and the ignition by means of a single signal source. This control device controls the injection and the ignition with a constant timing ratio independently of the engine speed. The start of the injection and of the ignition are separated by a predetermined time interval which is measured by a time delay device which operates independently of the engine speed. This method for determining the ignition time is applied over the entire rotational speed range and it should be possible to apply it even in internal combustion engines which have higher rotational speeds than those of motor vehicles.
U.S. Pat. No. 4,621,599 discloses a method for injecting a quantity of fuel into a combustion space of an internal combustion engine and igniting it, in which method in each case a predetermined quantity of fuel for ignition is injected into the combustion space at a specific angular position of the upper dead center, and this quantity for ignition is ignited at the same time or with a specific delay. An additional quantity of fuel is injected with a time interval with respect to the quantity for ignition, which quantity is somewhat reduced in the case of a low load. In the case of a relatively large load, an additional quantity of fuel is injected in advance of the upper dead center by a specific angular position. As a result of the provision of a plurality of injection pulses, the injection method is very complex in its configuration and requires considerable control expenditure.
The invention is based on the object of providing a method for controlling the ignition in internal combustion engines, which method can be implemented with simple means and yet can be applied over the entire rotational speed range of the internal combustion engine, brings about a high degree of smooth running particularly at low rotational speeds, especially when idling, gives rise to a very good level of efficiency and considerably reduces the emission of pollutants.
The object is achieved by means of a method having the features of claim 1. Advantageous refinements are defined in the subclaims.
According to the invention, the ignition time is triggered below a predetermined load threshold or rotational speed threshold if a predetermined time period has passed since the triggering of the injection process. In order to carry out the ignition, it is not the angular position of the crankshaft in the region below the predetermined load threshold or rotational speed threshold which is measured, but rather the expiry of a predetermined time period since the triggering of the injection process. This ensures that at the ignition time the fed-in fuel/air mixture is in a predetermined state in which it can easily be ignited. The measuring inaccuracies occurring at low rotational speeds ( less than 2000 rpm) when the ignition time is being determined as a function of the crankshaft position are avoided, since the ignition time is specified only as a function of the injection process so that very smooth idling is achieved.
Above the load threshold or rotational speed threshold the ignition time is determined in a manner known per se by measuring a predetermined angular position of the crankshaft, as a result of which a specific relationship between the crankshaft angle of the upper dead center and the ignition angle can be maintained precisely in a simple way.
The method according to the invention can be implemented using very simple technical means and achieves sensational synchronism in idling mode so that a two-stroke internal combustion engine can idle smoothly even at a rotational speed of 180 rpm.
The method according to the invention can be applied in all internal combustion engines having injection devices in which the fuel, or the fuel/air mixture, is fed to the combustion space with a predetermined time sequence, i.e. the injection process is a time-invariant process, the time period of which depends only on a few parameters, for example the quantity of fuel injected per injection process.